1. Field of the Invention
The present invention relates to a failure diagnostic system for an exhaust pressure increasing device, and more particularly to a failure diagnostic system for an exhaust pressure increasing device which is applied to exhaust flow control intended for reducing the emission of exhaust gas.
2. Description of the Related Art
As a technique for reducing the emission of unburned substances such as HC and CO and exhaust gas substances such as NOx, an exhaust gas emission reducing technique which utilizes reaction caused by a catalyst has been known. According to this technique, the flow of exhaust gas is controlled to prevent unburned substances such as HC from being emitted before the catalyst is activated.
It is known that exhaust flow control is provided in e.g., an exhaust pressure increasing device provided in an exhaust channel of an internal combustion engine. The exhaust pressure increasing device is comprised of an exhaust throttle valve, an actuator which actuates the exhaust throttle valve, and so forth; the passage area of the exhaust channel is varied by changing the angle of opening of the exhaust throttle valve so as to increase the exhaust pressure of an exhaust pressure increasing part located upstream of the exhaust throttle valve. The increase in exhaust pressure promotes the reaction of the unburned substances and oxygen in an exhaust system which extends from a combustion chamber to the exhaust channel, and reduces the emission of the unburned gas substances and realizes quick activation of the catalyst.
Since the exhaust pressure increasing device is important for reducing the emission of the exhaust gas substances and realizing quick activation of the catalyst as described above, it is necessary to promptly and properly cope with a failure in the exhaust pressure increasing device.
To determine whether the exhaust pressure increasing device has failed or not, it can be envisaged that an exhaust system pressure sensor (exhaust pressure sensor) is disposed upstream of the exhaust pressure increasing part so that whether a failure has occurred or not can be determined according to whether the exhaust pressure detected by an exhaust pressure sensor lies within a predetermined range or not. A technique relating to a construction in which an exhaust pressure sensor is provided which detects the exhaust pressure in an exhaust channel has been proposed, for example, in Japanese Laid-Open Patent Publication (Kokai) No. 8-210123 (e.g., paragraph [0016] and FIG. 1)).
According to this technique, the exhaust pressure sensor which detects the exhaust pressure is disposed at an end of a port (cooling pipe) which branches off from the exhaust channel of an exhaust system. With this arrangement, whether the exhaust increasing apparatus has failed or not can be determined by detecting the exhaust pressure of the exhaust pressure increasing part.
By the way, the exhaust pressure sensor is required to have a resistance to heat since it has to detect the pressure of high-temperature exhaust gas at a position upstream of the exhaust pressure increasing part. Moreover, the exhaust pressure sensor is in communication with the exhaust channel so as to detect the pressure of the exhaust pressure increasing part, and hence the exhaust pressure sensor is also required to have a resistance to corrosion caused by condensed water in exhaust gas. Particularly, in an engine for which fuel with sulfur with a high concentration is used, the exhaust pressure sensor is likely to corrode due to a high acidity of the condensed water. Therefore, the exhaust pressure sensor disposed in the exhaust system needs to be specially configured to have a resistance to heat and a resistance to corrosion, and is expensive as a result.
Here, to satisfy the requirements such as a resistance to heat, it can be envisaged that the exhaust pressure sensor is not disposed directly in the exhaust channel through which high-temperature exhaust gas passes, but is disposed at the end of the cooling pipe which branches off from the exhaust channel as in the above described prior art. According to this arrangement, exhaust gas taken out from the exhaust channel can be cooled, but the cooling pipe where the exhaust pressure sensor is disposed is required to have a length sufficient to withstand high-temperature exhaust gas, and thus the cooling pipe as well as the exhaust pressure sensor itself needs to be configured to have a resistance to heat, which leads to cost increase. Further, the problem may arise in terms of the vibration-resisting property of the cooling pipe connected to the exhaust channel.
As described above, if the exhaust pressure sensor disposed in the exhaust system detects the pressure of the exhaust pressure increasing section so as to determine whether the exhaust pressure increasing section has failed or not as in the prior art, the problem arises in terms of the detecting accuracy of the exhaust pressure sensor laid under unfavorable conditions.